Sodium metasilicate, i.e., sodium silicate having a molar ratio SiO.sub.2 :Na.sub.2 O of about 1, is frequently used in cleaning products such as detergents and dishwashing agents. The silicate inhibits corrosion on glass, glazing, and metal parts, and also functions as an alkali generating agent. Generally, granules of pentahydrate are used, which granules dissolve quickly in water. However, many products contain metasilicate in such a high amount that a very high alkalinity is obtained, which renders the products corrosive and unpleasant to handle.
Accordingly, the first requirement for products containing sodium silicate is to lower their alkalinity. Lower alkalinity but the same protection against corrosion can be obtained by the use of sodium silicates having a higher molar ratio SiO.sub.2 :Na.sub.2 O, for example disilicates with a ratio of about 2. However, higher ratios also can be employed, i.e., up to a ratio of 3.3.
Preference is also given to an amorphous product over a crystalline product. The preparation process for crystalline products is more elaborate than for amorphous products, in view of the extra high temperature treating step which has to be applied. Also, it is more difficult to compact crystalline sodium silicate, and the resulting product has a high amount of dust.
Furthermore, a compacted sodium silicate is required for ease of handling of the product by producers and detergent manufacturers. Powders are more difficult to handle, in view of dust.
For use in highly concentrated cleaning products it also is highly desirable for the silicate to have the highest possible bulk density. However, this bulk density should not have an adverse effect on the dissolution of the compacted sodium silicate in water.
In addition to the above, the compacted sodium silicate serves as a carrier for, e.g., surfactants and other washing aids, such as polycarboxylates. This requires a high porosity of the compacted silicate, to give a high loading capacity.
Finally, the compacted sodium silicate is required to have a high mechanical strength. As used herein, mechanical strength is defined as the inverse of the weight percentage of dust with a particle size below 400 .mu.m, resulting from a milling operation of compacted silicate with a particle size between 400 and 1000 .mu.m, as will be described hereinafter. A high mechanical strength, therefore, results in a low percentage of dust, which is an advantage when handling the product after preparation but especially avoids dust problems during various after treatments, such as adsorption of the above-mentioned washing aids, and introduction into the cleaning composition.
Compacted sodium silicate is known from U.S. Pat. No. 3,931,036. However, the disclosed product lacks the required porosity and mechanical strength. Furthermore, it is prepared according to a spray-drying technique known in the art. Accordingly, the outlet temperature required in the process of the present invention has not been applied in the process of this disclosure.
Compacted sodium silicate is also known from EP-A-0 486 078 and EP-A-0 486 079. Again, the products disclosed do not have the required porosity and mechanical strength. Also, as mentioned above, a normal sprayrying technique is applied, requiring an outlet temperature substantially lower than that of the process of the present invention.
The object of the present invention is to provide a compacted amorphous sodium silicate having the above-mentioned properties. More particularly, the invention concerns compacted sodium silicate having a molar ratio SiO.sub.2 :Na.sub.2 O from 1.6 to 3.3, characterized in that the silicate has the following properties:
a bulk density of above 500 g/l, PA1 a porosity of above 190 ml/kg, and PA1 a dust percentage of below 40 wt %. PA1 a bulk density of above 500 g/l, PA1 a porosity of above 190 ml/kg, and PA1 a mechanical strength such that the dust percentage is below 40 wt %. PA1 spray-drying an aqueous solution of sodium silicate with a molar ratio SiO.sub.2 :Na.sub.2 O from 1.6 to 3.3, PA1 applying a water glass solution to the spray-dried powder, PA1 subjecting the water glass treated powder to a compacting step, applying a pressure below 100 bar, and PA1 crushing the resulting flakes having a thickness of above 4 mm to obtain the compacted sodium silicate, PA1 wherein during the spray drying step an outlet temperature above 114.degree. C. is applied. PA1 spray-drying an aqueous solution of sodium silicate with a molar ratio SiO.sub.2 :Na.sub.2 O from 1.6 to 3.3, applying an outlet temperature above 114.degree. C., PA1 applying a water glass solution to the spray-dried powder, PA1 subjecting the water glass treated powder to a compacting step, applying a pressure below 100 bar, and PA1 crushing the resulting flakes having a thickness of above 4 mm to obtain the compacted sodium silicate.